1. Field of the Invention
A field-effect transistor employs a nitride semiconductor, and specifically a field-effect transistor includes an active region such as a channel formed by stacking the semiconductor layers over a substrate via an underlayer such as a buffer layer.
2. Description of the Related Art
A field-effect transistor (FET) having a stacked layer of nitride semiconductors has been predicted to be a high output power semiconductor device that can operate at high frequencies with high breakdown voltage. Some of these field-effect transistors, for example, a Metal Semiconductor FET (MESFET) and a HEMT High Electron Mobility Transistor (HEMT) are described in, for example, Japanese Unexamined Patent Application Publication Nos. H-11-297713 and 2001-247399, and International Publication No. WO 03/007383.
An example of an HEMT formed from a nitride semiconductor is shown in FIG. 3. The HEMT shown in FIG. 3 includes a carrier transit layer (channel layer) 303 made of GaN and a carrier (electron) supply layer 305 made of AlGaN that are stacked over a sapphire substrate 301 via a buffer layer 302. A source electrode 311, a gate electrode 312, and a drain electrode 313 are respectively disposed over a top surface of the carrier supply layer 305. When a voltage is applied to the drain electrode 313, electrons are supplied from the source electrode 311 to the carrier supply layer 305, and from the carrier supply layer 305 to the carrier transit layer 303. The supplied electrons form a two-dimensional electron gas (2DEG), which performs as a channel in the carrier transit layer 305 in a region along the carrier supply layer 305. With high mobility, the electrons again flow in the channel formed in the carrier transit layer 303 and through the carrier supply layer 305, and the electrons further flow into the drain electrode 313. Since the range of the channel changes according to the voltage applied to the gate electrode 312, the amount of electric current that flows between the source electrode 311 and the drain electrode 313 can be controlled by adjusting the voltage applied to the gate electrode 312.
However, in a conventional FET formed from a nitride semiconductor, a certain amount of leakage current generates, resulting in poor breakdown voltage. That is, the electric current flows in a region other than the channel such as an underlayer, which should be insulated. As a result, the electric current flowing between the source electrode and the drain electrode cannot be sufficiently controlled even the voltage applied on the gate electrode is adjusted.